1. An illustrated guide to
the buffer cache
Selena Deckelmann
Based on Greg Smith’s “Inside the Buffer Cache”
http://www.westnet.com/∼gsmith/content/postgresql

2. Table
Databases Table
Table
Databases are made of tables...

3. $PGDATA/base/ Table
A table is just a directory...

4. $PGDATA/base/ Table
Table
Table
You can have more than one table.

5. $PGDATA/base/ Table
Table
Table Made of Made o
f 8K
File 8K
8K
File
8K
8K
File
8K
File 8K
...
... Up to 1 GB
Files are made up of 8K blocks.*
* you can change that at compile time.

6. shared_buffers

7. shared_buffers
ptr ptr ptr ptr ptr ...
An array of pointers to 8K blocks
(and some other stuff)
shared_buffers is configured at database start.
You have to restart Postgres to change it.

8. shared_buffers:
the other stuff
FREE FREE FREE FREE FREE FREE
In the beginning...

9. shared_buffers:
the other stuff
FREE FREE FREE FREE FREE FREE
456 12 100 612 32 409

10. shared_buffers:
the other stuff
X X X X X X
456 12 100 612 32 409

11. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409

12. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
Status FLAGS FLAGS FLAGS FLAGS FLAGS FLAGS

13. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
Status PINNED flags flags flags flags flags
{ }
UPDATE mytable (status)
VALUES(‘my cat is the best ever’)
where user = ‘selenamarie’;

14. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
Status PINNED DIRTY flags flags flags flags
{ COMMIT;
}

15. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
Status PINNED DIRTY OTHER OTHER OTHER OTHER
{ BLAH BLAH BLAH;
}

16. shared_buffers:
the other stuff
X X X X X X
Tag base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
Status PINNED DIRTY OTHER OTHER OTHER OTHER
Usage 1 5 5 4 3 0

17. shared_buffers:
the lifecycle
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 100); }

18. shared_buffers:
the lifecycle
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
PINNED DIRTY PINNED OTHER OTHER OTHER
1 5 5 4 3 0
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 100); }

19. shared_buffers:
the lifecycle
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 101); }

20. shared_buffers:
the lifecycle
Who’s it gonna be?
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
PINNED DIRTY OTHER OTHER OTHER OTHER
1 5 5 4 3 0
Eviction! { myPgProcess calls:
BufferAlloc(‘base/file1’, 101); }

21. shared_buffers:
the lifecycle
Who’s it gonna be?
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 409
PINNED DIRTY OTHER OTHER OTHER OTHER
0 5 5 4 3 0
Scan the cache!
Usage != 0,
so decrement.
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 101); }

22. shared_buffers:
the lifecycle
Who’s it gonna be?
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 Usage == 0,
456 12 100 612 32 409 Goodbye!
PINNED DIRTY OTHER OTHER OTHER OTHER
0 4 4 3 2 0
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 101); }

23. shared_buffers:
the lifecycle
Who’s it gonna be?
X X X X X X
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1
456 12 100 612 32 101
PINNED DIRTY OTHER OTHER OTHER PINNED
0 4 4 3 2 1
{ myPgProcess calls:
BufferAlloc(‘base/file1’, 101); }

24. shared_buffers:
optimizations*
X X X X X X X ...
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...
PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
{ }
Result is greater
than: SELECT pg_relation_size(‘mytable’);
shared_buffers/4 VACUUM mytable;
* Version 8.3 and later

25. shared_buffers:
optimizations
X X X X X X X ...
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...
PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
256K ring
buffer allocated!
{ SELECT pg_relation_size(‘mytable’);
VACUUM mytable; }
* Version 8.3 and later

26. shared_buffers:
optimizations VACUUM
Safe during
OVERSIMPLIFICATION WARNING!
X X X X X X X ...
base/file1 base/file2 base/file1 base/file3 base/file1 base/file1 base/file2 ...
456 12 100 612 32 101 891 ...
PINNED DIRTY OTHER OTHER OTHER PINNED OTHER ...
0 4 4 3 2 0 1 ...
256K ring
buffer allocated!
{ SELECT pg_relation_size(‘mytable’);
VACUUM mytable; }
* Version 8.3 and later